Under Attenuated Beer – Causes & How to Fix

by Daniel on June 4, 2014

Last October I wanted to try a new beer style I hadn’t attempted before, and one that was at a higher difficulty level than most of the ales I’ve done before – I brewed a scotch ale. The brew day requires a longer boil to help develop the flavors from carmelization, but otherwise is pretty much the same. The tricky part comes with fermentation. Using WYEAST 1728 Scottish Ale (2 smack packs to make sure the yeast had a good start), I had my apartment’s ambient temperature set to 61 degrees F. The colder temperatures are desired with a scotch ale to help more slowly develop the refined flavors that are desirable in this type of beer, where higher temps can produce too many esters which impart undesirable fruity & overly sweet characters to the beer.

Unfortunately, we had a cold snap that my apartment’s thermostat couldn’t keep up with while I was away for a weekend, and little did I know at the time, the slight but quick temperature drop was just enough to put my yeast in hibernation mode before completing fermentation. Fast forward four months or so, after a transfer to my secondary fermenter (carboy), to bottling day. My buddy and I painstakingly washed more bottles than we needed, which is always a good idea when you’ve already got everything set up. Every time you bottle, you always want to test your product to make sure it’s good. I siphoned off a portion of my product to measure with a hydrometer, and to my “horror” discovered that the gravity was 1.04 (started at 1.062) – nowhere near completion at 1.01.

Immediately my mind went back to the entire process, and I looked at my excel spreadsheet brew log. What could have happened? What can cause under attenuation? One cause can be not pitching enough yeast (especially with high original gravity beers like imperial stouts, barleywines, belgian tripels, etc). The second most common, which I arrived upon since I’d pitched two fresh smack-packs, was an unexpected temperature change. Even a slight change of a couple degrees, if it occurs quickly enough, can cause your yeast to drop into hibernation – especially when fermentation is already relatively low, making the yeast work harder and slower.

Do I dump the beer? I’d rather not. A good general rule is, if the beer isn’t overtly ruined, why not try and do something to recover? Because I was ready for bottling, I had made a simple syrup already. I decided to get another smack pack and try to finish the fermentation.

When you have a partially attenuated beer, it’s not easy for yeast to get started as they are inhibited some by the existing alcohol in the beer which they aren’t accustomed to it. It’s pretty much impossible to reawaken the existing yeast that’s gone into hibernation, so get yourself a new culture. The other thing I did to help my yeast was to increase my fermentation temperature. This was a compromise because for my scotch ale I knew this would likely produce some esters that I didn’t want. But perhaps it would still turn out to be a potable beer.

This time, to be more certain, I put my fermentation bucket into my fermentation chamber and set it at 71F. Bubbling started nicely, although a little slow. I let it sit in there for 2 months just to be sure. This past weekend I decided to crack open the bucket and test the gravity and lo and behold, 1.01 final gravity! I kegged the rest and tasted the gravity sample. It definitely picked up more ester character than I would have liked, but I think it’ll still be an okay beer. I’m not going to be entering it in competitions, but after carbonation if I feel it’s decent enough to share, at least I made something of it. I’d love to hear your questions and comments. Happy brewing!

Thanks Dan. It turned out okay. It wasn’t a perfect “to style” scotch ale (the higher fermentation temp the second time around produced more esters, giving fruity & banana type notes that aren’t supposed to be in a scotch ale. However, it tasted fairly good and my friends and I still ended up drinking all of it. Better result than dumping it!